This image, taken in 2005, shows the immediate aftermath of a collision between a probe and comet Tempel 1 during NASA's Deep Impact mission. Scientists are now pondering a similar mission to a different object, 2015 BZ509, an asteroid near Jupiter which may have come from another star. Credit: NASA, JPL-Caltech and UMD

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From time immemorial, people gazing up at the night sky have dreamed of reaching out to touch the stars. But today we know that even the closest ones are so far away that light itself, the fastest thing known, takes several years to make the trip. The dream of such a visit seems as remote as the stars themselves—unless, perhaps, the stars somehow send emissaries to us.

Remarkably, that may be happening. Last year astronomers spotted a curious body they called ‘Oumuamua, streaking through the solar system too fast to be caught in the sun’s gravitational clutches; its trajectory confirmed it was an interstellar voyager, tossed out from its unknown system long ago to drift alone through the galaxy. ‘Oumuamua was the first of its kind to be observed, and now it may have another newfound counterpart much closer to home.

Researchers Fathi Namouni of Côte d'Azur Observatory in France and Helena Morais of São Paulo State University in Brazil say they have identified an interstellar asteroid that, rather than passing through, somehow settled down in our solar system. If confirmed, the discovery would open the possibility for robotic missions to visit and investigate a piece of another planetary system without ever leaving our stellar home. The findings were published Monday in the Monthly Notices of the Royal Astronomical Society.

“This shows the solar system is home to objects which were born around other stars,” Morais contends. “Thus, matter in other star systems could influence the evolution of our own solar system.” That, in turn, would complicate the scenarios scientists have assembled to explain some of our solar system’s most fundamental mysteries, such as the detailed timing and mechanics of planet formation, the delivery of water and organic molecules to Earth, and even the genesis of life. Rather than originating here, for instance, could life have hitchhiked in after forming elsewhere in the galaxy? Each time astronomers find a space rock that could be an interstellar immigrant, the need grows to take such far-out ideas seriously.

Backwards in Orbit, Backwards in Time

Discovered in late 2014 via the Pan-STARRS telescope in Hawaii, the object was provisionally dubbed 2015 BZ509. Scarcely anything is known about it at all, save for its size (about three kilometers wide) and orbit, which is almost identical to Jupiter’s. Almost, that is, save for one important detail—BZ509’s orbit is backwards, or retrograde, meaning it moves in the opposite direction of the prograde orbits of almost everything else circling the sun.

This backward motion goes against the fundamental counterclockwise spin our solar system inherited from more than 4.5 billion years ago, when our star and its planets first coalesced from a whirling disk of gas and dust. BZ509’s retrograde status is doubly strange near Jupiter because the giant planet’s gravitational influence would tend to make such an orbit short-lived, tossing the object out of the solar system or down toward the sun within millennia. Such interactions with giant planets are thought to be the main way small objects become interstellar in the first place. Perhaps, scientists initially speculated, BZ509’s odd orbit was due to its origins in the Oort cloud—an enormous reservoir of comets ejected by Jupiter and the other giant planets to the outer limits of the solar system, where they can be disturbed by passing stars. If this were the case with BZ509, however, we would be seeing it at an exceedingly special and rare point in its history: a brief moment of illusory orbital stability that will soon decay into chaos.

Yet no comet-like emissions have been detected from BZ509, and it is in resonance with Jupiter, synchronized to periodically swoop within about 175 million kilometers of the giant planet’s cloud tops—just close and often enough for Jupiter to regularly provide gentle tugs that keep its orbit stable for at least a million years. So how did BZ509 get there? And how long will it stay?

To find out, Namouni and Morais built a virtual time machine. They used a supercomputer to simulate the possible motions of a million digital clones of BZ509, each with slightly different orbital parameters reflecting astronomers’ limited knowledge of the real object’s orbit. Running their simulation backwards across billions of years of virtual time, they watched almost all the clones succumb to orbital instability. But at 4.5 billion years ago—rewinding the clock to when the solar system was less than 100 million years old—they saw 46 clones remaining, twirling away in stable orbits.

If BZ509 has indeed been in its retrograde orbit that long, it would predate the generally accepted timeline for the Oort cloud’s formation by more than three billion years, making the cloud an implausible source. Instead, Namouni and Morais reason, our sun’s gravity must have captured the object after it was somehow ejected from the grasp of a nearby neighboring star. Such a star would likely have been a sibling of our sun born alongside us in a “stellar nursery”—a nebula filled with star-forming gas and dust. Galactic motions long ago scattered this progeny far and wide, but it seems remnants may linger on our doorstep today.

“We did not expect that the asteroid would remain bound to Jupiter and that it would hang on in there for 4.5 billion years, but it did!” Namouni says. “Since the asteroid's orbit was right there as it is now—in retrograde and in the same resonance with Jupiter—it can't have been born in the solar system.”

An Interstellar Origin—Maybe

Namouni and Morais’s argument hinges on the notion that, on average, the odds are against us being lucky enough to witness any asteroid at some extremely special moment in its history. Namouni expresses this as a simple rule: “If we have two possible orbits for the asteroid—one stable over the age of the solar system, and one that is unstable after say 10 million years—then it is the stable one that represents the real physical orbit.”

But critics say this “nothing special” reasoning cuts both ways. Namouni and Morais have not yet modeled the probability of an interstellar asteroid being captured into a stable retrograde orbit with Jupiter, and for that matter have not provided any detailed, step-by-step scenarios for how exactly this could occur. “The probability of a capture like this is certainly quite low. But how low is it?” asks Scott Tremaine, an astrophysicist at the Institute for Advanced Study in Princeton who did not take part in the work. “For a realistic flux of interstellar material, does capture happen often enough to make the discovery of such an object plausible?” In terms of probability, an interstellar origin for BZ509 could be exceedingly special in comparison to other rare but likelier alternatives, Tremaine says. He speculates BZ509 could have emerged through “some combination of a close encounter with Jupiter and a collision” with another object.

Not all the possible scenarios are so prosaic. Konstantin Batygin, an astrophysicist at Caltech, also not involved with the work, suggests a wild possibility: Planet Nine, a hypothetical 10-Earth-mass undiscovered world far beyond Pluto that Batygin proposed in 2016 with fellow Caltech astronomer Mike Brown, would inevitably “pollute” the solar system with retrograde objects much like BZ509 as it drifts through the outer solar system. “So do you need to draw [BZ509] from the interstellar medium? No—Planet Nine would give it to you for free,” Batygin says. “The interstellar interpretation is one possibility; Planet-Nine-induced dynamics provide an alternative.”

Additionally, Tremaine says, the idea that BZ509 simply came from the Oort cloud cannot be easily dismissed, in part because there is no good reason to believe the object was actually captured 4.5 billion years ago or earlier (before the Oort cloud’s estimated formation circa one billion years ago). “If these objects are captured at a steady rate and all survive, yes, you would expect a typical one to have arrived a few billion years ago,” he says. “But if most of the captured objects are lost on much-shorter timescales as expected, the typical ones we’d see would be younger.”

Even if BZ509 came from the Oort cloud, Namouni counters, it could still have originated from beyond the solar system—because the Oort cloud itself could in theory be largely composed of comets captured from interstellar space. An interstellar origin for the Oort cloud could only have occurred prior to 4.5 billion years ago, when our newborn sun was still embedded in its stellar nursery—a decidedly non-standard scenario, but one consistent with Namouni and Morais’s dating of BZ509 to the dawn of the solar system.

The Brick and the Truck

Certainty about BZ509’s origins can only come from follow-up studies. This would specifically entail searches for additional interstellar interlopers, which Namouni and Morais predict should pile up in orbits roughly perpendicular to the plane in which the known planets orbit the sun. Then, Morais says, “it would be very interesting to analyze the composition of this object and other immigrant candidates,” to seek out chemical signatures of their births around alien suns and subsequent journeys through the spaces between the stars. “The more we identify interstellar asteroids in the solar system,” Morais adds, “the more we understand their influence on its evolution.”

For BZ509 in particular, she says, “a robotic interplanetary mission is highly desirable and feasible, because the asteroid is not far away from Earth.” And, in fact, such missions are already in embryonic phases of planning. After ‘Oumuamua stormed through the solar system in 2017, scientists began studying how such objects could be intercepted and studied.

The most realistic approach is also the cheapest and easiest, as laid out in a paper by Yale University astrophysicists Darryl Seligman and Greg Laughlin. Given sufficient early warning by ground-based telescopes, a probe could be launched into the path of any future ‘Oumuamua-like interstellar asteroid as it reaches its closest point to Earth. The asteroid would be moving too fast for the probe to match its velocity, but the probe could eject a hefty “impactor” to strike the object as it flies by—“like throwing a brick in front of a speeding truck,” Seligman says. The impact would eject a plume from the object’s interior that both the probe and Earth-bound observers could then study telescopically. (Incidentally, NASA already performed a mission much like this in 2005, when its Deep Impact spacecraft raised and studied a plume from the ordinary comet Tempel 1.)

Because of BZ509’s retrograde motion, Seligman says, the Deep-Impact-style approach would be ideal. Sending a spacecraft to gently land upon or orbit the object would be extremely difficult, as such a probe would have to burn huge amounts of fuel to cancel out its prograde velocity. But the relative velocities of a prograde “brick” in the path of a retrograde “truck” would be huge, enough to easily produce an enormous plume.

“What’s nice about BZ509 is that it’s not leaving our solar system, whereas with an ‘Oumuamua-like object you just get one shot,” Seligman says. “So you can really take your time and plan out the best point in its orbit to hit it.” Even so, he says, any BZ509 mission is unlikely to get off the ground anytime soon, given the uncertainties about the object’s true origins. “The case for BZ509 to be interstellar looks good and compelling, but people will still probably find other ways to explain its retrograde orbit—it’s not quite the same as having an object shooting through our solar system like ‘Oumuamua did.”

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